Just to add another point of view, split starts can be a real nighmare when there is a problem with the first group. The rules of start dictate that if the first group gets a green, then the second group gets a green, no matter what shape the field is in. Then it's up to a start judge, if one is designated, to apply penalties to offending drivers. If there is an issue with the first group in the first turn, hopefully there is enough time to bring out a yellow or waving yellow to warn the drivers, who may or not be watching the starter (how many drivers have radios and crews to tell them when the starter throws the green?). There have been very painful consequences as the result of the second group charging into turn one with 50 - 70% track blockage.

Just another point of view.